Extraction apparatus



June 29, u" WAINWRIGHT 2,682,452

EXTRACTION APPARATUS (Itforneg June 29, 1954 L. WAINWRIGHT 2,682,452

EXTRACTION APPARATUS K Gttorneg Patented June 29, 1954 EXTRACTION APPARATUS Lawrence Wainwright, Brooklyn, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Application March 16, 1951, Serial No. 216,043

3 Claims. l This invention relates to .apparatus for continuously bringing into contact and separating two liquids, at least partially immiscible witheach other and of different specic gravities, particularly for the purpose of countercurrent extraction. p

Apparatus iorbringing two immiscible liquids into Hcontact with each other has been used for many yearsl for the purpose of extracting from one of the liquids a valuable component dissolved therein. This type of process isparticularly useful and efective'in casesiwhere distillation or evaporation of the` solution containing the valuable component is expensive and a favorable distributionof the valuable component .can be obtained with a selective solvent immiscible with the solution. In the description of thisinvention the term solvent isused to describe the liquid with which the valuable component is extracted; `the term solution is used to describe the liquid from vwhich .thedesired .component is to be extracted; the term extraclfwill be used to describe the solvent afterit has been in contact with thedsolution; the term .rafnate will be used to describe the treated solution.

One indispensable requisiteof extraction apparatus is intimacy vof .contact between the phases to attain equilibriumas rapidly as possible. It has been found diflicult to obtain both the necessary completemixing of the twophases and the complete separationof the phases 4after mixing in a continuous, countercurrent manner. Generally, a compromise is made between the intimacy of Contact between the phases and the desire for countercurrent owand complete separation. Most recently, mucheffort hasbeen expended in the design of rotating centrifugal countercurrent contactors. vThe principal difficulties with these is their initial cost: and their rather complicated mechanioalstructure. 4`Such extractors have a `tendencytowdevelop vleaks through seals and bearings and to become clogged by any solids in theliquid` stream. Where extractors are to. be used under conditions which makethemaintenance of the equipment impracticable, for lexample in` handling rav dioaotivev fluids, the `problems yraised by, vthese diiculties are multipliedl It is an object ofthis invention yto provide a simple, eilicient, continuous extraction apparatus; Another 'objecty of the invention is to provide a compact `countercurrqerlitv .extractionmapparatus in which themonly moving partsare pumps. It is a lsomewhat morespecicobjectofthis invention to provide an extraction apparatus in which the immiscible. liquids are separated at least partially by centrifugal. force yet which uses no rotating equipment at all or none otherv than .ordinary liquid. pumps. A more specic object of the invention is to provide a multistage, countercurrent extraction apparatus in which the most intimate contact is obtained between the solvent and the solution to. attainequilibrium rapidly and yet in` which there are no moving parts other. than ordinary liquid pumps. A still further object of the invention is to provide a multi-stage countercurrent extrac-l tion apparatus oi the type described. wherein each of the `stages is an individual extractor, yet wherein the iiow between the stages is simply and effectively controlled. Anotherobject is to provide an extractor having little or no tendency to become clogged withsolids in the liquid streams. Still another object is to provide an extractor which is capable of taking a considerableoverload without flooding. Other objects will become apparent from the following specication. v

The elements in each stage of the present extractor yare arranged in a particular sequence with respect to the direction of flow of the two immiscible liquids. These elements are, in order, pumping means for solvent and solution; mixing means for solvent and solution; a conduit having a rst portion whose diameter is such that the mixture of solvent and solution flows through it turbulently to transfer solute from solution to solvent; a stationary helical passage formed in a second portion of the conduit whereby extract is separated from raiiinate; and means for partitioning the extract and the raffinate.

The objects and advantages of the apparatus will become apparent from the following` description taken with reference to the accompanying iigures in which:

Figure l is la diagrammaticrepresentationof a single-stage apparatusv according to this invention for bringing into contact and separating a Figure 3 is. a ,diagrammatic plan View of the partitioning means shown in Figure 2 to illustrate particularly the. manner in which` interstage now of liquids is equalized.

Figure 4 is a diagrammatic plan view, mostly in section, of a compact multi-stage extractor operating on the principles of the apparatus illustrated in the foregoing figures.

Figure 5 is a diagrammatic side elevation of the extractor shown in Figure 4 and is partly cut away to show the arrangement of the various parts of each extraction stage.

The following description will be limited to a specific embodiment of the invention. However, it is to be understood that this description is illustrative only and that it is not intended to limit the invention thereby.

Reference will first be made to the apparatus of Figure l. In Figure l, there are shown two containers I0 and l2, one containing a solution and the other a solvent. For convenience it will be assumed that container I0 holds the solution and container l2 holds a solvent which is heavier than the solution. The solution is drawn from container I0 through pipe I4 containing iiowcontrol valve I6 into the intake of a pump I8. The pump may be of any convenient type which produces the proper flow rates and pressures. A small gear pump has been used for this purpose and found satisfactory, but centrifugal pumps and other types may also be used. The

- pump I8 discharges the solution through pipe containing a rotameter 22 or other flow measuring device (such as an orifice, venturi, etc.) and through one nozzle 24 of jet mixer 42.

In a similar manner the solvent in container I2 is withdrawn through pipe 26 containing flowcontrol valve 2t by means of a pump 3l! which discharges through pipe 32 containingy a rotameter 34 into the second nozzle 3G of the jet mixer 42. The nozzles 24 and 36 both discharge in a small mixing chamber 38 so that the liquid streams impinge one upon the other. The combination of the two nozzles 2.4 and 3S discharging into a small chamber 38 at one end thereof, the chamber having an outlet port 44 at the other end thereof is referred to as the jet mixer 42. It has been found convenient to use an angle of between about 60 and 120 between the axes of the jet nozzles. However, this angle may be varied and the choice depends upon such factors as the rates of flow of the liquids, their tendency to form an emulsion, the sizes of the nozzles 24 and 36 and of the chamber 38 and similar factors. It is preferable also that one of the nozzles be withdrawn slightly with respect to the other so that one of the liquid streams impinges on the other. While this type of mixer is preferred because of the intimacy of contact obtained, other varieties of continuous mixers may be used. For example, both of the liquids may be metered into a single pump which performs both the pumping and mixing functions. While this is desirable in reducing the amount of equipment employed, the extent of mixing is more difcult to control. A relatively stable emulsion may be formed because of small changes in ow or the unavoidable introduction of an impurity which acts as an emulsifying agent. This difficulty can be avoided with the jet mixer which enables more accurate control.

Alternatively, a gas, steainor liquid-actuated eductor or jet pump may be used to pump the solvent and solution. However, the use of steam both heats and dilutes the liquids and can only be used where the presence of the added water is not objectionable. With a liquid-actuated eductor, one of the solutions and solvent is pumped through a jet nozzle and the pumped liquid entrains the other liquid. Such a system replaces the two rotary pumps and jet mixer with a single rotary pump and jet. However, this system of pumping and mixing is extremely difficult to control to avoid the formation of a stable emulsion.

The jet mixer 42 discharges the mixture of liquids through its outlet port 44 into an elongated cylinder or tube 40. The diameter of this tube is extremely important since it is here that the major part of the transfer of solute takes place to the solvent from the solution. The di ameter of this tube is required to be such that the mixture of liquids flows through the tube in turbulent flow. The term turbulent flow is used in its engineeringI sense in contrast to streamline or Poiseuille flow. Whether the flow is turbulent or streamline depends upon the Reynolds number which in turn depends upon the diameter of the tube, the linear velocity, the density of the mixture of liquids and the viscosity of the mixture of liquids. The theory and methods of calculation are adequately described in standard engineering texts, for example, The Chemical Engineers Handbook, McGravi/HilL New York, 1941. The required diameter of the tube 40 can easily be calculated from the rates of flow of the solution and solvent and their physical properties. The relative rates of flow are determined from the distribution coefiicient of the solute between solvent and solution and from the economic design of the particular extraction process. The length of transfer tube 40 depends upon 'the time required for the solution and solvent to reach equilibrium.

The transfer tube 4D discharges the mixture of rafnate and extract through the inlet port 42 of the helical separator 50 which comprises a stationary, generally tubular member 44 having an outlet port at `48 at the end opposite the inlet port and lled with an elongated screw member 4E. The liquid mixture of raffinate and extract flowing into the separator 55 ows along the threads 52 of the screw member through a helical passage 53 formed by the wall of the tubular i member 44 and the threads 52 and shank 54 of the screw member. It is desirable that the dimensions of the helical passage be chosen so that the rate of flow (and therefore the centrif ugal force) is as great as possible. However, the velocity of the mixture flowing through the helical passage should not be so great that mixing conditions are maintained in the helical passage. The optimumvelocity for any particular process can be determined by experiment.

The flow of a mixture of liquids through this helical passage causes the solvent phase to be separated at least partially from the solution by centrifugal force. That is to say, that the dispersion of one of the light and heavy liquids in the other, as formed in the mixing chamber 3S and maintained through the transfer tube 49, is broken in the helical separator 50 and the dispersed phase coalesces at least partially. The separated phases then flow together through pipe 54 into a decanter 5E where the phases can be partitioned completely. After settling by gravity,

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and I8, respectively, through nozzles 36 `andw24, respectively, into a smallwmixing-chamber- 38; The -two. streams of` solvent and solution discharged from the nozzles `36 and 24- inpinge one upon the otherin the chamber 68 `to disperse one of them in the other. The mixture of solvent and solution then. passes .throughitransfer tubeM 40 in turbulent flow-to effect the major part of transfer of. the solute `from the `solution to the solvent.` The mixture is discharged from the transfer tube-.40 into-the helicalfseparator 56 wherein the mixture ows through a helical pa.,- sage 53 formed by an elongated screw member 46 and the inner wall of the tube 44. The centrif ugal force developed in the helical passage causes the dispersed phase to coalesce at least partially and the separated mixture of extract and raiiinate are partitioned in a decanter 56. It will be noted that the transfer tube 46 and the separator 56 form a continuous conduit between mixing chamber 38 and the decanter 56. One portion of this conduit has a diameter such that the mixture of liquids iows therethrough. in turbulent flow, and a second portion is lled with the screw member 46vto form a helical passage with the wall of the second portion of the conduit. It will be noted also that the solvent and solution flow cocurrently through the single-stage apparatus.

In Figures 2 and 3, the arrangement of a plurality of stages (four in the embodiment shown) in a countercurrent extractor is illustrated diagrammatically. The functions and structures of the corresponding parts are identical with those described with reference to Figure 1 for a single stage.

The four stages shown are interconnected through a continuous decanter or partitioning means |68. The continuous decanter comprises a box-like vessel |61 divided into six interconnected compartments: settling chambers |56a, |56b, |56@ and |56d, corresponding to the four stages, A, B, C and D, and decanter chambers |96 and |14 outwardly of the settling chambers for partitioning continuously the light and heavy liquids respectively. Y

The settling chambers |56a, b, c, d, are separated from each other by pairs of spaced, upright baflles |66ab, |66bc and li'icd, the reference letters corresponding to the adjoining settling chambers. Each ofthe baflies |66 is sealed to the floor and only one wall of the vessel |61 so that the chambers |56 are interconnected through passages llllab, |bc and llllcd between the pairs of baiiles |66ab, l66bc and |66cd, respectively. The interconnections between the settling chambers |56 within continuous decanter |62 are made to equalize the interstage pumping rates. These interconnections also prevent iiooding of the extractor under any but the most severe overload conditions.

The decanter chambers |96 and |14 at opposite ends of vessel |61 are employed to separate continuously the extract and raliinate. The heavy liquid decanter chamber |14 into which the light liquid feed is admitted through pipe 262 (asl presently described) is separated. from settling chamber |56d by means of a pair of spaced, upright bailes |12. In a manner similar to baliies |66, the baffles |12are separated by a passage |16 connecting decanter chamber |14 and adjoining settling chamberV |56d. However, in order to separate the lighter from the heavier of the `two liquid phases, the baifles |12 are raised above the iioor of vessel |61 to provide a passage |66 through which the heavy liquid product of chamber |56d flows into the `decanter chamber |14. To partition the -heavy liquid from the light liquid, the decanter chamber |14 is divided by an upright baille |82 sealed `to opposite walls of the vessel |61 with a space below the baffle providing a passage |84.- As long as the interface between the liquids is maintained above the bottom edge of` baiiie |82, only heavy liquid can flow through passage |84. Between baille |62 and the end wall |13 of vessel |61, there is a Weir or dam |86 over whichthe heavy liquid flows from the passage |66. The dam |36 serves to maintain a head of heavy liquid between it and baille |62 to maintain the interface leveland thereby to prevent ow of light liquid through passage |84. The heavy liquid is removed from chamber |14 through outlet pipe |88 in end wall |13.

At the opposite end of the vessel |61 adjacent chamber |5606, means are provided for decanting the light liquid i from the heavy liquid. The light liquid decanter means comprises a decanter chamber |96 formed between a pair of upright spaced bales |92 separating chamber |96 from chamber |56a 4and the end wall |66 of the vessel |61. The space between the baffles |922 forms a connecting passage 94 between chamber |96 and chamber |56a similar in structure, pur" pose and effect to passages |16 and |16. The height of the baffles |62 is such that light liquid product from chamber |56a may now in volume over the baffles in a manner similar to a weir or dam. However, since heavy liquid feed is admitted to chamber |96 (as will be presently described) and since chambers |96 and |560. are interconnected, a dam |96 is provided between end wall |69 and baffles |92. The light product liquid ilows over the dam |96 and is removed through outlet pipe |98 in end wall |66.

The arrangement of the equipment in the four stages and the interstage connections are shown principally in Figure 2. The light liquid feed is delivered by pump 20|) through pipe 262 into the heavy decanting chamber |113 between baffle |62 and baies |12. The heavy liquid product is res1 moved from chamber |14 through passage |65-, over dam |86 and out through pipe |86 as already described. The light liquid feed is removed from chamber |14 through pipe I4 by means of pump ||8d which discharges in stage D through pipe |2|Jd into the jet mixer |42d. Here the light liquid feed is mixed with heavy liquid from settling chamber |5|ic` of the next stage, C, as presently described. The mixture passes successively through the transfer tube |46d and the helical separatorr |5611` and is discharged through inlet port 20401 into the chamber |56d. l-iee the heavy liquid settles out from the light liquid and flows through passage into decanter chamber |14.

The light liquid is withdrawn from chamber |5611 through pipe |6201 by means of pump ||8c of stage C. The pump ||8c discharges light liquid through pipe |200 into jet mixer |420 where it is mixed with heavy liquid withdrawn from chamber |561) of the next stage, B, as presently described.` The mixture flows from mixer |424: successively through transfer tube |400 and helical separator |5|ic and is discharged through inn let port 204e into chamber |560 where the light and heavy liquids separate. The heavy liquid is withdrawn from chamber |560 through outlet port 206e in the chamber floor' by means of pump |3001 of stage D and is discharged through pipe |32d into the jet mixer |42d. l

The light liquid in chamber |56c is withdrawn through pipe |62c by means of pump H81) of stage B and is discharged through pipe |2012 into the jet mixer |4212. Here the light liquid is mixed with heavy liquid pumped `from chamber |56@ of stage A, as presently described. The mixture of liquids formed in mixer I 4217 flows successively through transfer tube |4017 and helical separator |5019 and is discharged through inlet port 2041) into chamber |5617. The heavy liquid is removed from chamber |561)` through pipe |5819- by means of pump |c (of stage C) which discharges through pipe [32C into the jet mixer |42c.

The light liquid separated in chamber |561) is i withdrawn through pipe |6211` by means of pump ||8a, of stage A and is discharged through pipe |20a into jet mixer |42a. In the mixer Edic the light liquid from stage Bis mixed with heavy feed liquid from decanter chamber |90 and the mixture flows successively through transfer tube Mila and helical separator [50a into chamber 156er. The heavy liquid separated in chamber i560. is withdrawn through conduit |5811, by means of pump |3011 of stage B and is discharged through conduit |321)` into jet mixer |421).

A portion of the light liquid in chamber |56a overflows the baffles |92 'into decanter chamber |90 where it is partitioned as the light product by overflowing the dam or weir |90, as a'lready described. The heavy liquid feed is pumped into the decanter chamber |90 through pipe 200 by means of pump 206 and is withdrawn from the chamber |90 through pipe |26 by means of pump |36a (of stage A) which discharges the heavy feed through pipe |32a into the jet mixer Ifsa.

In the multi-stage apparatus, it is preferable that all of the pumps be powered by a single prime mover. As described with respect to the single-stage apparatus of Figure 1, the jet mixer and pumps of each stage may be replaced by a pump which serves not only to pump but also to mix the heavy and light liquids. However, it is preferred to use individual pumps and jet mixers.

In tests of a four-stage apparatus constructed as shown in Figures 2 and 3, hexone was used to extract acetic acid from water, and a hexone solution of acetic acid was washed with water to extract the acetic acid. Small gear pumps were used to pump the liquids. The axes of the nozzles of the jet mixers |42 were set at about 60 to each other and the nozzle which discharged the hexone into the mixing chamber was slightly withdrawn with respect to the water nozzle so that the hexone stream impinged upon the water stream. The mixing chambers were approximately 1" in diameter and of an inch long and the nozzles were about 1/8 in diameter. It was found that under the conditions used the hexone phase was usually dispersed in a continuous aqueous phase. Each transfer tube consisted of 4 feet of 1A Saran tubing which is approximately g of an inch in inside diameter. Each helical separator consisted of a brass tube with an inside diameter of containing a brass screw member with two acme-style screw threads to the inch and with an outside diameter of 3A. The shank of the screw member was M3 in diameter and the threads were each thick. This apparatus, when continuously treating l to 1.5 gallons per minute, was found to be equivalent to approximately four theoretical stages. That is, in each stage of the apparatus complete equilibrium was attained as well as complete separation.

In Figures 4 and 5, there is shown an extremely compact eighteen-stage extractor operating on the principles already described. In Figures 4 and 5, the partitioning means 268 comprises an annular vessel 261 positioned vagainst the wall of a generally cylindrical housing 21|. The vessel 261 is divided into settling chamber 256 by pairs of spaced upright bales 266 and has decanter chambers 290 and 214 for partitioning the light and heavy liquids respectively. The chambers 1 290 and 214 are positioned adjacent each other with a fluid-tight wall 320 between them. The inlet pipe 308 introduces the heavy liquid feed into the light-liquid decanting chamber 290 similar to chamber for partitioning the light liquid product. A pair of baffles 292 similar in structure and function to baffles |92 separates decanting chamber 290 from the adjacent settling chamber 256. The light liquid product is removed from chamber 290 through pipe 298. The light liquid feed is introduced through pipe 302 into the decanter chamber 214 similar to decanter chamber |14. The heavy liquid product is removed through pipe 288 from the heavy liquid decanter chamber 214. Decanter chamber 214 vis separated from adjacent settling chamber 256 by a pair of upright spaced baffles 212, identical in function and structure to baffles |12. The arrangement of the baies, conduits and dams in chambers 256, 214 and 290 is the same as that previously described and shown in Figures 2 and 3 with respect to chambers |56, |14 and |90, the chambers 256 being similarly interconnected by passages 210 between the pairs of baffles 266.

At the axis of the annular vessel 261, there is a pump housing 3|2 containing a plurality of pumps mounted on a single shaft 3|0 and powered by a single prime mover (not shown). The pumps are serially arranged within the housing in two groups with the heavy liquid pumps together below the light liquid pumps. The light and heavy liquid feed pumps (equivalent to pumps 200 and 4206 of Figure 2) may also be included within housing 3 l2. Light and heavy liquid feed are withdrawn from chambers 214 and 290 through conduits 2|4 and 226 respectively.

In each stage, a heavy liquid pump withdraws heavy liquid from a port 306 in the oor of a I settling chamber 256 through a pipe 258 and discharges the liquid through a pipe 232 into the jet mixer 242 of the next adjacent stage upstream with respect to the flow of heavy liquid through the extractor. The light liquid deliv- 1 ered through pipe 220 to that mixer 242 is withdrawn through a pipe 262 from a chamber 256, two chambers removed from the chamber supplying heavy liquid to mixer 242. The jet mixer 242 discharges through a transfer tube 240 and a i helical separator 250 in succession and into the chamber 256 between the chambers from which the light and heavy liquids respectively were Withdrawn. The countercurrent piping connections between stages are identical to those already described with reference to Figures 2 and 3. That is to say that into each of the chambers 256 (fed through a jet mixer 242, a transfer tube 240 and a helical separator 250) there is introduced a stream consisting of heavy liquid from the next preceding stage and light liquid from the next succeeding stage.

The eighteen-stage extractor shown in Figures 4 and 5 is about five feet in diameter and 2.5 feet high, exclusive of the prime mover for the pumps. The extractor is capable of treating approximately 8 gallons per minute. This is to be compared with an equivalent packed column having a diameter of about 1 foot and a height of about 30 to 50 feet. The advantages are manifest.

Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described, it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sense.

I claim:

1. Multistage continuous liquid-liquid contact apparatus for treating Ia solution containing a solute with a solvent at least partially immiscible with the solution, said solution and solvent having different densities, said apparatus comprising a horizontally arranged annular container having spaced baffles therein positioned to divide said container into a number of interconnected compartments corresponding tothe number or" stages, each of said compartments having a pair of Vertically spaced upper and lower outlet connections and an intermediate inlet connection, a series of transfer pumps, each of a plurality of the comlower outlet connection and upper outlet connection respectively of the two compartments adjacent to said one compartment, the first pump associated with said one compartment being the third pump of the next preceding compartment and the third pump associated with said one compartment being the first pump of the next succeeding compartment, said 'pumps being located substantially at the central axis of said annular container whereby the conduits interconnecting said pumps and said compartments are of substantially equal length.

2. Multistage Contact apparatus as claimed in claim 1 and wherein the conduits interconnecting the mixing devices and compartments contain elongated screw members delining with the conduit walls helical passages to give the liquid mixture flowing therethrough a whirling motion and thereby cause the disperse phase of the mixture to coalesce at least partially.

3. Multistage contact apparatus according to claim 1 wherein the pumps of said series are mounted on a common shaft aligned with the central aX-is of said annular container.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,756,026 l-luii Apr. 29, 1930 2,076,126 Guinot Apr. 6, 1937 2,192,094 Moore Feb. 27, 1940 2,250,976 Van Dijck July 29, 1941 2,381,760 Latham Aug. 7, 1945 2,432,308 Goodyer Dec. 9, 1947 

1. MULTISTAGE CONTINUOUS LIQUID-LIQUID CONTACT APPARATUS FOR TREATING A SOLUTION CONTAINING A SOLUTE WITH A SOLVENT AT LEAST PARTIALLY IMMISCIBLE WITH THE SOLUTION, SAID SOLUTION AND SOLVENT HAVING DIFFERENT DENSITIES, SAID APPARATUS COMPRISING A HORIZONTALLY ARRANGED ANNULAR CONTAINER HAVING SPACED BAFFLES THEREIN POSITIONED TO DIVIDE SAID CONTAINER INTO A NUMBER OF INTERCONNECTED COMPARTMENTS CORRESPONDING TO THE NUMBER OF STAGES, EACH OF SAID CONPARTMENTS HAVING A PAIR OF VERTICALLY SPACED UPPER AND LOWER OUTLET CONNECTIONS AND AN INTERMEDIATE INLET CONNECTION, A SERIES OF TRANSFER PUMPS, EACH OF A PLURALITY OF THE COMPARTMENTS OF SAID CONTAINER BEING CONNECTED TO A FIRST, SECOND AND THIRD PUMP OF SAID SERIES, A SERIES OF MIXING DECICES EACH OF WHICH IS CONNECTED TO THE DISCHARGE SIDES OF ONE OF SAID FIRST PUMPS AND ONE OF SAID SECOND PUMPS AND TO THE INLET CONNECTION OF ONE OF SAID COMPARTMENTS, OF CONDUIT CONNECTING THE LOWER OUTLET CONNECTION OF SAID ONE COMPARTMENT AND THE SECTION SIDE OF ONE OF SAID THIRD PUMPS, CONDUITS CONNECTING THE SUCTION SIDES OF SAID FIRST AND SECOND PUMPS WITH THE LOWER OUTLET CONNECTION AND UPPER OUTLET CONNECTION RESPECTIVELY OF THE TWO COMPARTMENTS ADJACENT TO SAID ONE COMPARTMENT, THE FIRST PUMP ASSOCIATED WITH SAID ONE COMPARTMENT, THE FIRST PUMP THIRD PUMP OF THE NEXT PRECEDING COMPARTMENT AND THE THIRD PUMP ASSOCIATED WITH SAID ONE COMPARTMENT BEING THE FIRST PUMP BEING SECCEEDING CONPARTMENT, SAID PUMPS BEING LOCATED SUBSTANTIALLY AT THE CENTRAL AXIS OF SAID ANNULAR CONTAINER WHEREBY THE CONDUITS INTERCONNECTING SAID PUMPS AND SAID COMPARTMENTS ARE OF SUBSTANTIALLY EQUAL LENGTH. 